Gmr sensor element and its use

ABSTRACT

A GMR sensor element is proposed, having a rotationally symmetrical positioning of especially eight GMR resistor elements which are connected to each other to form two Wheatstone&#39;s full bridges. This GMR sensor element is especially suitable for use in an angle sensor for the detection of the absolute position of the camshaft or the crankshaft in a motor vehicle, particularly in the case of a camshaft-free engine having electrical or electrohydraulic valve timing, of a motor position of an electrically commutated motor, or of detection of a windshield wiper position, or in the steering angle sensor system in motor vehicles.

FIELD OF THE INVENTION

The present invention relates to a GMR sensor element according to themain claim, as well as its use.

BACKGROUND INFORMATION

The giant magneto-resistive effect (GMR effect) may be utilized, in theform of so-called spin-valve structures (“spin-valves”) forangular-position sensing. This is described, for example, in PCTInternational Publication No. WO 00/79298 or in European PublishedPatent Application No. 0 905 523 A2.

GMR spin-valves are made up in essence of two ferromagnetic thin filmshaving a resulting magnetization m₁ or m₂, which are separated from eachother by a nonmagnetic thin film lying in between. The electricalresistance R(α) of such a layer system then shows a cosine-type functionof the angle α between the direction of magnetization m₁ and thedirection of magnetization m₂ of the form:R(α)={overscore (R)}−Q5·ΔR _(GMR)·cos(α)

In this context, the maximum relative resistance changeΔR_(GMR)/{overscore (R)} designates the GMR effect, and typicallyamounts to 5% to 10%. GMR spin-valve layer systems, by the way, areusually deposited with the aid of cathode sputtering of the respectivematerials, and then structured using customary photolithography methodsand etching techniques.

What is essential for the intended spin-valve function is a rigid, atleast approximately not changeable direction of magnetization m₁ of thefirst ferromagnetic layer, of the so-called reference layer (RL),because of a magnetic field, acting from outside on the layer system,that is to be detected particularly with regard to its direction and/orstrength, and a direction of magnetization, m₂, of the secondferromagnetic layer, of the so-called free layer (FL) or detectionlayer, that orients itself slightly, at least approximately parallel tothe outer magnetic field. In order to achieve both of these, on the onehand, the two ferromagnetic layers are magnetically decoupled by asufficient thickness of the nonmagnetic intermediate layer, of theso-called nonmagnetic layer (NML), of typically a few nanometers, andthe magnetization of the reference layer (RL) is fixed or “pinned”, forinstance, by an additional, directly adjacent antiferromagnetic layer, aso-called natural antiferromagnet (AF), and by its mutual magneticcoupling by exchange interaction.

This is shown schematically in FIG. 1 a, where the GMR layer system orGMR sensor element is under the influence of a magnetic field of amagnetic transducer. One may achieve a further improved stabilization ofthe reference magnetization by adding an additional so-called syntheticor “artificial” antiferromagnet (SAF). This SAF, corresponding to FIG. 1b, is made up of two ferromagnetic layers that is stronglyantiferromagnetically coupled via a nonmagnetic intermediate layer. Theferromagnetic layer of these two, which lies directly next to or on thenatural antiferromagnet AF, is designated as the pinned layer (PL),since its magnetization MR is fixed or “pinned” as a result of thecoupling with the natural antiferromagnet (AF). The second ferromagneticlayer of the SAF, whose magnetization MR is oriented opposite to that ofthe pinned layer (PL) as a result of the antiferromagnetic coupling, isused as reference layer (RL) for the abovementioned GMR spin-valve layersystem.

In order to extract the angle-dependent useful signal, in a GMR sensorelement according to the related art, four spin-valve resistanceelements are connected together to form a Wheatstone's bridge circuit(Wheatstone's full bridge), such as by using an aluminum thin film trackconductor. The maximum signal amplitude is obtained by, as in FIG. 2,oppositely oriented reference magnetizations MR of the bridge resistorswithin the half bridges and similarly oriented reference magnetizationsMR of the resistors lying diagonally in the full bridge.

As a rule, a GMR angle sensor also has a second full bridge of GMRresistors, whose reference directions, as shown in FIG. 2, are rotatedby 90° to the ones of the first bridge. Signal U_(sin) made available bythe second full bridge is thereby phase-shifted by 90° with respect tothe signal of the first full bridge U_(cos).

By arctangent formation or corresponding algorithms (such as the CORDICalgorithm) one then determines, from the two cosine-shaped orsine-shaped bridge signals U_(sin), U_(cos), the angle α, that issingle-valued over a full 360° revolution, to the direction of an outermagnetic field B.

The different reference magnetization directions according to FIG. 2are, for instance, implemented in that the individual GMR bridgeresistors are heated locally to a temperature T above the blockingtemperature (Néel temperature) of the antiferromagnetic layer (AF), butbelow the Curie temperature of ferromagnetic layers (PL, RL) as in FIG.1 a or 1 b, so that the antiferromagnetic spin order in theantiferromagnetic layer is canceled, and thereafter they are cooled inan outer magnetic field of a suitable field direction. In the renewedformation, taking place in this context, of the antiferromagnetic order,the spin configuration resulting from the exchange interaction at theinterface of antiferromagnetic layer (AF) and adjacent ferromagneticlayer (PL) is frozen. As a result, the direction of magnetization of theadjacent ferromagnetic layer (pinned layer PL) is fixed. The localheating of the GMR bridge resistors may take place, for example, withthe aid of a brief laser pulse or current pulse. The current pulse maybe driven, in this context, directly by the GMR conductor structureor/and an additional heating conductor.

In the case of known GMR angle sensors, reference magnetization MR ofthe individual bridge resistors is selected to be either parallel orperpendicular to the direction of the strip-shaped structured GMRresistor elements. This is used to hold the influence of the shapeanisotropy to a low value. Furthermore, the strip-shaped structured GMRresistor elements are preferably aligned in parallel within a fullbridge according to FIG. 2. This is used to suppress a signalcontribution because of a superimposed anisotropic magnetoresistiveeffect (AMR effect). The AMR signal contribution is based, in thiscontext, on a function of the electric resistance of the angle α betweenthe current direction and the magnetization direction of the form:R(θ)={overscore (R)}+Q5·ΔR _(AMR)·cos(2·θ)

If, on the other hand, the GMR resistors are implemented within a halfbridge and having orthogonal alignment of their GMR strips, as is thecase, for example, in FIG. 10 in PCT International Publication No. WO00/79298, then the AMR signal contribution is even maximally favored.That acts in a worsening manner on the angular accuracy of the GMR anglesensor.

SUMMARY OF THE INVENTION

For the reasons mentioned, therefore, known GMR angle sensors have norotationally symmetrical positioning of the bridge resistors. Rather,both full bridges are usually positioned laterally next to each other.Therefore, as a result of the lacking rotational symmetry, a heightenedsensitivity of known sensors comes about with respect to the directionalinhomogeneity of the transducer field, i.e. of the magnetic field actingfrom the outside, as well as with respect to temperature gradients.

Because, in known GMR angle sensors, the pinning direction or referencedirection within a bridge resistor always has a fixed angle to the stripdirection, these sensors do not further offer the possibility ofcompensating for shape anisotropy-conditioned influences on the pinningbehavior and such disadvantages on the accuracy of the angular sensing.

By contrast, for an angular sensor that records over 360°, rotationalsymmetry in the sensor design is a great advantage, so that one does notobtain additional, direction-dependent angular error contributions, justbecause of an asymmetry in the positioning of the individual GMRresistor elements.

Therefore, because of the rotationally symmetrical positioning of theGMR resistor elements in the two Wheatstone's bridges, both a reducedsensitivity with respect to field direction inhomogeneities andtemperature inhomogeneities is achieved and an undesired AMR signalcontribution is suppressed, and, furthermore, the shape anisotropyinfluence on the pinning behavior and the angle sensing accuracy of theGMR sensor element is reduced. It is also especially advantageous if,besides the rotationally symmetrical positioning of the GMR resistorelements in the two Wheatstone's bridges, an interleaved positioning ofthe resistors with each other is selected. This leads to a furtherreduced sensitivity to field direction inhomogeneities and temperatureinhomogeneities.

The suppression of the interfering AMR signal contribution is achievedby an additional subdivision of every single GMR bridge resistor elementinto two equal halves, or partial bridge resistors, having GMR stripdirections that are oriented orthogonally to one another. Thisparticularly also leads to an increase in angular measurement accuracy.It is also advantageous, in this connection, that, because the directionof the strip-shaped structured GMR resistor elements (“GMR stripdirection”) is selected for respectively one of the two partial bridgeresistors to be parallel, and is selected for respectively the other ofthe partial bridge resistors to be perpendicular to the pinningdirection or reference direction, an averaging comes about of theinfluence of pinning directions parallel and perpendicular to the stripdirection within each of the GMR bridge resistor elements. The pinningbehavior is then, in turn, identical for all two-part GMR bridgeresistor elements (average of two parts, in each case). In this case,the two bridge output signals U₁, U₂ advantageously also have a 45°phase shift with respect to each other.

If the GMR resistor elements have a pinning direction or a referencedirection which has been selected to be at least approximately less than45° to the direction of the strip-shaped structured GMR resistorelements, this leads advantageously to an identical pinning behavior ofthe individual GMR resistor elements, i.e. especially to an improvedsignal stability and long-term stability of the GMR sensor element. Inthis case, the two bridge output signals U₁, U₂ also have a 45° phaseshift with respect to each other.

Bridge output signals U₁, U₂, that are phase-shifted by any desiredangle ν to each other, ν being preferably 45° or around 45°, mayfinally, advantageously, be imaged by a coordinate transformation toorthogonal signals having a 90° phase shift. From the latter, the angleα, being sought after, to the direction of outer magnetic field B may bedetermined, by arctangent formation or a corresponding algorithm, suchas the CORDIC algorithm. Beyond this, the coordinate transformationoffers the advantage that production-caused fluctuations in the phasedifference of the two bridge signals U₁, U₂ are able to be adjustedduring the imaging on the orthogonal signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a simplified GMR spin-valve layer construction having twoferromagnetic layers RL and FL that have magnetizations m₁ and m₂, onenonmagnetic intermediate layer NML and an antiferromagnetic layer AF.The latter is used for fixing (pinning) reference magnetization m₁. Inaddition, a magnetic transducer is provided for generating an outermagnetic field B. The angle α designates the angle between the fielddirection or magnetization direction of the free ferromagnetic layer(FL), and thus also the direction of the outer magnetic field B in theplane of the GMR sensor element, and the reference magnetizationdirection.

FIG. 1 b shows a GMR spin-valve layer system having a naturalantiferromagnet AF and an additional synthetic antiferromagnet SAF, aswell as an additional nonmagnetic intermediate layer NML and aferromagnetic free layer FL.

FIG. 2 shows an equivalent circuit diagram for an angle sensor elementbased on the GMR effect having two full bridges (Wheatstone's bridgecircuits), the reference magnetization MR being oriented pairwiseopposite within the two bridges, and being rotated from bridge to bridgeby 90° with respect to each other. The direction of referencemagnetization MR further is parallel or perpendicular to the directionof the individual, strip-shaped structured GMR resistor elements, whichare constructed, for example, as in FIG. 1 a or 1 b. This “stripdirection” is represented by the indicated strip set within theindividual GMR resistor elements. Besides that, in FIG. 2 the directionof an outer magnetic field B is indicated, which, together with areference direction, subtends the angle a that is is to be measuredusing the GMR sensor element. The reference direction or zero directionis, in this context, specified by the selection of the referencemagnetization directions in the two full bridges, of which one isdesigned as a sine full bridge and one as a cosine full bridge.

FIG. 3 shows a rotationally symmetrical device of meandering,interleaved GMR bridge resistor elements 1/1 to 1/4 (bridge 1) and 11/1to 11/4 (bridge 11). In this context, the directions of the referencemagnetization (see arrows marked in FIG. 3) in bridge 1 are in each caseoriented at less than 45° to the direction of the individualstrip-shaped structured GMR resistor elements, and the referencemagnetization directions in bridge 11 are rotated in each case by 45°with respect to those in bridge 1. Besides that, in FIG. 3 the directionof an outer magnetic field B is indicated, which, together with areference direction, subtends the angle α that is to be measured usingthe GMR sensor element. The reference direction or zero direction is, inthis context, specified by the selection of the reference magnetizationdirections in bridge 1 and bridge 11, bridge 1 being supposed to supplya cosine-shaped signal curve over the angle α.

FIG. 4 shows an equivalent circuit diagram to the layout of the GMRsensor element according to FIG. 3. The pinning or referencemagnetization direction MR is, in this context, in each case oriented atless than 45° to the GMR strip direction, which is, analogously to FIG.2, once more indicated by the strip set drawn in inside the individualGMR resistor elements, and in bridge 11 is rotated additionally by 45°with respect to that in bridge 1. A strengthening of the AMR signalcontribution comes about as a result of strip directions of theresistors of each half bridge that are orthogonal to one another.

FIG. 5 a shows GMR sensor output signals U₁ and U₂ having a 45° phasedifference according to a pinning direction or reference magnetizationdirection MR less than 45° to the strip direction, corresponding to FIG.3 and 4.

FIG. 5 b shows correspondingly transformed GMR sensor signals U_(cos)and U_(sin) that are orthogonal to each other, having a 90° phasedifference. The AMR signal contribution is not shown in FIG. 5 a andFIG. 5 b. The direction of outer magnetic field B in degrees, i.e. theangle α, is in each case plotted on the x axis in FIGS. 5 a and 5 b,while on the y axis there is plotted, in FIG. 5 a the GMR sensor outputsignal in mVolt/Volt, and in FIG. 5 b the transformed GMR sensor signalin mVolt/Volt.

FIG. 6 shows a rotationally symmetrical, at least approximately circularor octagonal, interleaved positioning of meandering GMR bridge resistorelements, a suppression of the AMR signal contribution having beenundertaken by subdividing each of the individual bridge resistorelements into two equal halves having strip directions that areorthogonal to one another.

FIG. 7 shows an equivalent circuit diagram to the GMR resistor elementsas in FIG. 6. Suppression of the AMR signal contribution is hereachieved by subdividing each bridge resistor element I/1, I/2 to II/4into two halves a and b, having GMR strip directions that are orthogonalto one another. The respective pinning magnetization or referencemagnetization MR is less than 45° to the respective GMR strip direction.The latter is indicated by the strip set drawn in within the individualGMR resistor elements.

FIG. 8 shows an equivalent circuit diagram to the layout of the GMRresistor elements according to FIG. 6, having pinning magnetizationdirections or reference magnetization directions MR, alternative to FIG.7, of less than 0° and 90° to the GMR strip direction at each of theindividual bridge resistors I/1, I/2 to I/4. The averaging of theinfluence of the pinning direction takes place here by pinning directionor reference magnetization direction, both parallel and perpendicular tothe GMR strip direction, within each two-part bridge resistor I/1, I/2to I/4.

DETAILED DESCRIPTION

a.) Rotationally Symmetrical Positioning

FIG. 3 shows a possible rotationally symmetrical positioning ofaltogether eight bridge resistor elements of two full bridges(Wheatstone's bridges). In contrast to AMR sensors, in which thereference direction is given by the current direction which is specifiedby the strip direction, in the case of the GMR angle sensor, thereference direction is specified by the direction of the magnetizationof reference layer (RL). In principle, the pinning direction orreference direction may, in this context, be selected as desired,however, in order to obtain the same pinning behavior for all bridgeresistor elements, in this case an orientation of the pinning directionor reference direction is selected that is less than 45° to the stripdirection. This is made even clearer in FIG. 4, where, besides the stripdirection (strip set inside the resistor symbols) the direction of thereference magnetization MR is also given.

b.) Imaging on Orthogonal Signals

In the case of a pinning direction or a direction of the referencemagnetization less than 45° to the GMR strip direction, according toFIG. 5 a the two bridge output signals U₁ and U₂ do not have the usualphase shift of 90°, but only a phase shift of 45°. These signals U₁ andU₂ may, however, be transformed in a simple manner to the orthogonal,cosine-shaped and sine-shaped signals according to FIG. 5 b. To do this,the following transformation is performed in a sensor evaluationelectronic device: $\begin{pmatrix}U_{\cos} \\U_{\sin}\end{pmatrix} = {{\begin{bmatrix}1 & 0 \\\frac{\cos(\varphi)}{\sin(\varphi)} & \frac{- 1}{\sin(\varphi)}\end{bmatrix} \cdot \begin{pmatrix}U_{1} \\U_{2}\end{pmatrix}} = ( \frac{\begin{matrix}U_{1} \\{{U_{1} \cdot {\cos(\varphi)}} - U_{2}}\end{matrix}}{\sin(\varphi)} )}$

In this equation, ν denotes the phase shift of the second bridge signalwith respect to the first bridge signal. This phase shift may, inprinciple, be selected as desired, but preferably a phase shift of 45°is set.

From the cosine-shaped and sine-shaped signals obtained with the aid ofthis transformation, according to FIG. 5 b, one may determine the angleα by arctangent formation or by applying a corresponding algorithm, suchas the CORDIC algorithm in the sensor electronic system:$\alpha_{mess} = {\arctan( \frac{U_{\sin}}{U_{\cos}} )}$

The implementation of this coordinate transformation further offers theimportant advantage that production-conditioned fluctuations in thephase shift of the two bridge signals U₁, U₂ are able to be detectedspecifically as to the sensor during imaging to the orthogonal signal(90° phase shift) and compensated for. To do this, for example in anoffset adjustment or amplitude adjustment of the signals U₁, U₂ at theend of a production line, this phase shift ν is also determined, forexample, using Fourier analysis of the two bridge signals U₁, U₂, and isstored in the sensor evaluation electronic system.

c.) Rotationally Symmetrical Positioning Having Suppression of the AMRSignal Contribution

The resistor positioning shown in FIG. 3 favors the AMR signalcontribution, since the GMR strip directions of the two bridge resistorsof each half bridge are orthogonal to each other. This disadvantage maybe avoided in that, according to the preferred, also rotationallysymmetrical positioning according to FIG. 6, one puts together eachbridge resistor from two equal halves having GMR strip directions thatare perpendicular to each other. Because of the series connection of thetwo partial resistors each having identical reference magnetization MR,the AMR component is then filtered out, while the GMR signal componentremains unchanged as a result of the identical direction of referencemagnetization MR in the case of both partial resistors. This situationis made clearer by the following relationship for a two-part GMR bridgeresistor element: $\begin{matrix}{{R(\alpha)} = {\underset{\underset{1\quad{Teilwiders}\quad\tan\quad d}{︸}}{\frac{1}{2} \cdot ( {\overset{\_}{R} - {{0.5 \cdot \Delta}\quad{R_{GMR} \cdot {\cos(\alpha)}}} + {{0.5 \cdot \Delta}\quad{R_{AMR} \cdot {\cos( {2\vartheta} )}}}} )} +}} \\{\underset{\underset{2\quad{Teilwiders}\quad\tan\quad d}{︸}}{\frac{1}{2} \cdot ( {\overset{\_}{R} - {{0.5 \cdot \Delta}\quad{R_{GMR} \cdot {\cos(\alpha)}}} + {{0.5 \cdot \Delta}\quad{R_{AMR} \cdot {\cos( {2( {\vartheta - {90{^\circ}}} )} )}}}} )}} \\{= {\overset{\_}{R} - {{0.5 \cdot \Delta}\quad{R_{GMR} \cdot {\cos(\alpha)}}}}}\end{matrix}$

In this equation, α denotes the angle between field direction andmagnetization direction of the free ferromagnetic layer (FL) and thereference magnetization direction; θ denotes the angle between the fielddirection or magnetization direction of the free layer (FL) and the GMRstrip direction of the first partial resistor. The strip direction ofthe second partial resistor is rotated by −90° with respect to the firstpartial resistor.

d.) Pinning Behavior

FIG. 7 makes clear the subdivision of the bridge resistors into twohalves in each case, having strip directions that are orthogonal to eachother, but having an identical reference magnetization direction MR. Inprinciple, the pinning direction and the direction of referencemagnetization MR may be selected as desired. However, an angle of 45° tothe respective strip direction is preferred, because thereby anidentical pinning behavior is achieved for all partial resistors.

Alternatively, one may also set a pinning direction or a direction ofreference magnetization MR which is oriented respectively parallel tothe strip direction for one of the two partial resistors andrespectively perpendicular to the strip direction for the other of thetwo partial resistors. It is true that thereby one achieves a differentpinning behavior in the case of the individual partial resistors,however, overall again an identical pinning behavior is achieved in thecase of each of the bridge resistor elements in the form of a seriesconnection of the two partial resistors.

Compared to known sensors, this selection of the pinning direction orreference magnetization direction yields the advantage that, inside eachbridge resistor element, via the different pinning behavior of paralleland perpendicular alignment of the pinning direction or the referencemagnetization direction to the GMR strip direction, an average valuecomes about.

The 360° GMR angle sensor described is especially suitable for thedetection of the absolute position of the camshaft or the crankshaft ina motor vehicle, particularly in the case of a camshaft-free enginehaving electrical or electrohydraulic valve timing, of a motor positionof an electrically commutated motor, or of detection of a windshieldwiper position, or in the steering angle sensor system in motorvehicles.

1-7. (canceled)
 8. A GMR sensor element, comprising: eight GMR resistorelements arranged in a rotationally symmetrical positioning andconnected to each other to form two Wheatstone full bridges.
 9. The GMRsensor element as recited in claim 8, wherein the GMR resistor elementsare interleaved.
 10. The GMR sensor element as recited in claim 8,wherein the GMR resistor elements are structured in strip form.
 11. TheGMR sensor element as recited in claim 8, wherein each GMR resistorelement of the Wheatstone full bridges is subdivided into two equallyconstructed halves having directions, of the GMR resistor elements thatare structured in strip form, that are orthogonal to each other.
 12. TheGMR sensor element as recited in claim 8, wherein the GMR sensor elementperforms a determinate measurement of an angle of an outer magneticfield with respect to a magnetization of a reference layer over 360°.13. The GMR sensor element as recited in claim 8, wherein the GMRresistor elements are situated at least approximately in one of circularfashion and octagonally.
 14. A method of using a GMR sensor elementincluding eight GMR resistor elements arranged in a rotationallysymmetrical positioning and connected to each other to form twoWheatstone full bridges, the method comprising one of: using the GMRsensor element in an angle sensor for detecting an absolute position ofone of: one of a camshaft and a crankshaft in a motor vehicle in acamshaft-free engine having one of electrical and electrohydraulic valvetiming, a motor position of an electrically commutated motor, and of awindshield wiper position; and using the GMR sensor element in asteering angle sensor system in a motor vehicle.